Wireless communication networks use reference signal transmissions to support a variety of key functions. In this regard, a “reference” signal provides receiving radio equipment with some type of reference information—timing, frequency, phase, etc.—that enables certain measurements by the receiving equipment. For example, cell-specific reference signals, also referred to as common reference signals or CRSs provide receiving radio equipment with a basis for estimating propagation channel conditions. Other physical-layer reference signals include so called positioning reference signals or PRSs, which are particularly contemplated for newer, more capable networks, such as those based on the 3GPP Long Term Evolution (LTE) standards.
Release 9 of LTE, for example, contemplates the use of PRSs to enable and improve the burgeoning host of positioning-dependent services that are or will be offered in such networks. That is, beyond the law enforcement and safety requirements associated with positioning mobile stations and other user equipment, there is an increasing range of positioning-dependent applications and services, all relying on the ability of these newer wireless communication networks to efficiently and accurately support positioning services.
Indeed, the possibility of identifying user geographical location in the network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, i.e. FCC E911 in the U.S.
In many environments, the position can be accurately estimated by using positioning methods based on GPS (Global Positioning System). However, contemporary networks also often have the possibility to assist UEs in order to improve the terminal receiver sensitivity and GPS startup performance (Assisted-GPS positioning, or A-GPS). GPS or A-GPS receivers, however, may not necessarily be available in all wireless terminals. Furthermore, GPS is known to have a high failure incidence in indoor environments and urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3GPP. Correspondingly, PRSs play a key role in OTDOA measurements.
With OTDOA, the receiving radio equipment measures the timing differences for reference signals received from multiple distinct locations. For each (measured) neighbor cell, the equipment measures Reference Signal Time Difference (RSTD), which is the relative timing difference between a neighbor cell and the reference cell. The position estimate for the receiving equipment is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the receiving equipment and the receiving equipment clock bias.
More particularly, to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed. Position calculation can be conducted, for example, by a positioning server (eSMLC in LTE) or by the receiving equipment, which often is an item of user equipment (UE), such as a mobile terminal or other type of portable communication device. The former approach is referred to as a UE-assisted positioning mode, while the latter is a UE-based positioning mode. As noted, LTE has introduced the use of new physical signals dedicated for positioning (PRSs), as defined in 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation Positioning Reference Signals.”
PRSs generally are transmitted from one antenna port from a given base station for a corresponding cell according to a pre-defined pattern. A frequency shift, which is a function of the physical cell identity or PCI, can be applied to the specified PRS patterns, to generate orthogonal patterns and modeling an effective frequency reuse of six. Doing so makes it possible to significantly reduce neighbor cell interference as measured on the PRS for a given cell. Interference reduction on the PRS measurements correspondingly leads to improved positioning measurements.
Even though PRSs have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the current LTE standard does not mandate using PRSs. Other reference signals, the earlier mentioned CRSs can, in principle, be used for positioning measurements.
If PRSs are used, they are transmitted in pre-defined positioning subframes grouped by several consecutive subframes, with NPRS subframes in each positioning occasion. The positioning occasions are recurring, e.g., repeated according to a defined periodic interval having a certain periodicity of N subframes. According to 3GPP TS 36.211, the standardized periods for N are 160, 320, 640, and 1280 ms, and the number of consecutive subframes NPRS that define each positioning occasion are 1, 2, 4, and 6.
Because OTDOA-based positioning requires that PRSs be measured from multiple distinct locations, a receiving radio apparatus (user equipment or other radio node in the network) may have to work with a wide range of received signal strengths, e.g., the PRSs received from neighboring cells may be significantly weaker than the PRSs received from a serving cell. Furthermore, without at least approximate knowledge of when particular ones of the PRSs are expected to arrive in time and what PRS patterns are being used (e.g., arrangements within the time-frequency grid of OFDM signal subframes), the receiving radio apparatus is obligated to perform PRS searching within potentially large time-frequency windows. That, of course, increases the processing resources and the time needed for making PRS measurements, and tends to lower the accuracy of the results.
To facilitate such measurements, it is known for the network to transmit assistance data, which includes, among other things, reference cell information, neighbor cell lists containing PCIs of neighbor cells, the number of consecutive downlink subframes that constitute a positioning occasion, and the overall transmission bandwidth used for PRS transmission, frequency, etc.
Further, it is known to mute PRSs, where “muting” means transmitting with zero power (or low power) at certain positioning occasions. Such muting applies to all PRS resource elements—i.e., defined OFDM subcarriers at defined symbol times—within the same subframe and over the entire PRS transmission bandwidth. However, to date, the 3GPP standards do not specify how muting is to be implemented, nor do they specify signaling for communicating muting information to UEs or other receiving equipment that might be making use of the PRSs being transmitted by a given cell or a given cluster of neighboring cells.
Certain muting arrangements, however, have been contemplated. One contemplated approach is to use random muting by cells. Here, each eNodeB decides whether or not to mute its PRS transmissions for a given positioning occasion (or occasions) according to some probability. In the most basic contemplation of this approach, each eNodeB (cell) in the network independently makes muting decisions (according to some defined probability), without any coordination between the cells. The probability used to make the muting decision is statically configured per eNodeB or per cell.
While this approach offers certain advantages in terms of simplicity on the network side, it leaves receiving radio equipment with the same burdensome processing tasks, as said equipment has no knowledge of the random muting operations. A further issue is the inability to know the optimal probabilities to use for making the muting decision, and the fact that such probabilities likely change in dependence on complex interrelationships between cells (varying geometry, etc.), and may even change depending upon times of day, etc.
Another approach relies on a limited set of muting patterns, and maps these patterns according to PCIs. This approach allows a UE or other radio receiver to determine when PRSs are transmitted (or muted) in any given cell of interest, based on receiving information regarding the association between muting patterns and PCIs—e.g., a table. However, this approach requires signaling the actual patterns or hard-coding them into the receiving equipment. That latter approach may not be practical for some types of equipment. Besides, the static nature of such mapping has its own disadvantages.
Another approach proposes sending UEs an indication of whether or not autonomous muting is activated. A Boolean indicator is transmitted for the reference cell and also all neighbor cells as a part of the assistance data. When the indicator is FALSE, the UE can avoid blind detection of PRS muting, optimize detection thresholds and thus improve the positioning performance. However, with the indicator set to TRUE, the UE still does not receive information indicating when and for which resource blocks (RBs) muting occurs, meaning that the UE still needs to blindly detect when PRS muting is used in each cell.